This works !
I must have missed this.
And, yes, remotely controlling it from VNC is very slow. I wish I could install kstars on android, but I read everywhere it is not possible.
Many thanks.

Hello,
I had to reinstall from scratch due to a faulty SD-Card.
I (think I) did everything as I did once, but obviously it is not the case : I cannot connect with vnc anymore.
I cannot se what I could have missed, and what to focus on.
The password is the same for everything but wifi - thus I cannot be wrong.
Is there a way to fix this without having to reinstall everything ?
I did some settings which i would like not to lose.
Edit : OK, I think I found my mistake. I am currently restarting from zero, so I hope I can come back and confirm in 4~5 hours.
Rgrds,
J.-Luc

Got it !
Stupid me : for some unknown reason, I systematically launched setupAstroPi3.sh instead of setupUbuntuSBC.sh
In fact, after the first install, I kept a copy of the important files and folders : .conkyrc, config.txt, .config, .indi, .local and .ZWO.
Then I made a batch that would allow me to reinstall with no effort.
This is where I typed the wrong script name, so everytime I did it again it was the wrong way.
It took three days to realize. I suddenly remembered that my first install did not use RealVNC, whereas it was running in my further attempts.
So, all is running fine again now and I am happy. But feeling really like Wile E. Coyote when he realizes that he is getting caught by the trap he had set for the Road-Runner.
Now, I have to make my GPS module synchronize the clock. Some more hunting.